Energy efficient production of co2 using single stage expansion and pumps for elevated evaporation

ABSTRACT

A method and a plant for producing liquid CO2 out of combustion flue gases wherein the flue gas is partially condensed in a single stage phase separation, the single stage phase separation comprising at least one heat exchanger ( 11, 17 ) and a separation drum ( 19 ), wherein the at least one heat exchanger ( 11, 17 ) is cooled by expanded offgas ( 23 ) and expanded liquid CO2 ( 3.3 ) and wherein a first part of the expanded CO2 ( 3.3 ) is separated after having passed the at least one heat exchanger ( 17 ) into liquid CO2 and gaseous CO2 in an additional separation drum ( 33 ), wherein the gaseous CO2 (3.4) and the liquid CO2 ( 3.5 ) of the additional separation drum ( 33 ) are expanded to a first pressure level (flag  7   d ′) wherein a second part of the liquid CO2 ( 3.6 ) of the separation drum ( 33 ) is expanded to a second pressure level (flag  7   e ′) for cooling the flue gas in the at least one heat exchanger ( 17 ).

The invention relates to a method and a device for the liquefaction ofthe CO₂ contained in the flue gases. The liquefaction of CO₂ out of fluegases has been known for quite a long time.

Most cryogenic methods for the production of CO₂ out of combustion fluegases use conventional separation schemes having two or more separationstages. In FIG. 1 such a prior art installation is shown as blockdiagram.

In the figures of this application the temperature and the pressure atvarious points of the flue gas stream as well as of the CO₂ areindicated by so-called flags. The temperatures and the pressuresbelonging to each flag are compiled in a chart in the following. It isobvious for a man skilled in the art that these temperatures andpressures are meant as an example. They can vary depending on thecomposition of the flue gas, the ambient temperature and the requestedpurity of the liquid CO₂.

In a first compressor 1 the flue gas is compressed. This compression canbe a multi-stage compression process with coolers and water separatorsbetween each compression stage (not shown) separating most of the watervapour resp. water from the flue gas.

In FIG. 1 the flue gas stream is designated with reference numeral 3.When being emitted by the first compressor 1 the flue gas has atemperature significantly higher than the ambient temperature and thenis cooled to approximately 13° C. by a first cooler 5. The pressure isapproximately 35.7 bar.

The moisture still contained in the flue gas stream 3 is freed fromwater by a suitable drying process e.g. adsorption dried in a drier 7and subsequently conveyed to a first separation stage 9. This firstseparation stage 9 comprises a first heat exchanger 11 and anintermediate separation drum 13. The first heat exchanger 11 serves forcooling the flue gas stream 3. As a result of this cooling a partialcondensation of the CO₂ contained in the flue gas stream 3 takes place.Consequently, the flue gas stream 3 enters the intermediate separationdrum 13 as a two-phase mixture. There the liquid phase and the gaseousphase of the flue gas stream are separated by means of gravitation. Inthe first separation drum the pressure is approximately 34.7 bar and thetemperature is −19° C. (cf. flag no. 5).

At the bottom of the intermediate separation drum 13 liquid CO2 isextracted and via a first pressure reducing valve 15.1 expanded to apressure of approximately 18.4 bar (cf. ref. No. 3.1). This results in atemperature of the CO2 between −22° C. and −29° C. (cf. flag no. 10).The partial CO2 stream 3.1 of the flue gas is heated and evaporated inthe first heat exchanger 11 by the flue gas stream 3. At the exit of thefirst heat exchanger 11 the partial stream 3.1 has a temperature ofapproximately 25° C. and a pressure of approximately 18 bar (cf. flagno. 11).

Following the second partial stream 3.2 being extracted at the head ofthe intermediate separation drum 13 it becomes clear that this partialstream 3.2 being extracted from the intermediate separation drum 13 in agaseous state is cooled in a second heat exchanger 17 and partiallycondensed. Afterwards this partial stream 3.2 being also present astwo-phase mixture is conveyed to a second separation drum 19. The secondheat exchanger 17 and the second separation drum 19 are the maincomponents of the second separation stage 21.

In the second separation drum 19 again a gravity-supported separationbetween the liquid phase and the gaseous phase of the partial stream 3.2takes place. In the second separation drum 19 there is a pressure ofapproximately 34.3 bar and a temperature of approximately −50° C. (cf.flag no. 6).

The gaseous phase in the second separation drum 19, the so-called offgas23, is extracted at the head of the second separation drum 19, expandedto approximately 27 bar in a second pressure reducing valve 15.2, sothat it cools down to approximately −54° C. (cf. flag no. 7).

In the figures the offgas is designated with reference numeral 23. Theoffgas 23 streams through the second heat exchanger 17 thereby coolingthe flue gas 3.2 in the counter stream.

At the bottom of the second separation drum 19 liquid CO₂ (c. f. ref.num. 3.3) is extracted and expanded to approximately 17 bar in a thirdpressure reducing valve 15.3, so that it reaches a temperature of −54°C. as well (cf. flag no. 7 a). This stream 3.3 as well is conveyed tothe second heat exchanger 17. In the second heat exchanger 17 a part ofthe liquid CO₂ evaporates and stream 3.3 is expanded to approximately 5to 10 bar in a fourth pressure reducing valve 15.4, so that at thispoint a temperature of −54° C. is reached (cf. flag no. 7 b) and thestream 3.3 is again conveyed to the second heat exchanger 17.

After the stream 3.3 streamed through the second heat exchanger 17, itagain is conveyed to the first heat exchanger 11. At the entrance of thefirst heat exchanger 11 this stream has a pressure of approximately 5 to10 bar with a temperature of −22 to −29° C. (cf. flag no. 14).

This stream 3.3 takes up heat in the first heat exchanger 11, so that atthe exit of same it has a temperature of approximately −7° C. with apressure of approximately 5 to 10 bar. The third stream 3.3 is conveyedto a second compressor 25 at the first compressor stage, whereas thestream 3.1 having a pressure of approximately 18 bar is conveyed to thesecond compressor stage at the three-stage compressor 25 shown in FIG.1.

Intercooler between the various stages of the second compressor 25 andan aftercooler for the compressed CO₂ are not shown in FIG. 1.

At the exit of the second compressor 25 the compressed CO₂ has apressure of between 60 bar and 110 bar with temperatures of 80° C. to130° C. In the aftercooler, which is not shown, the CO₂ is cooled downto ambient temperature.

If necessary the CO₂ can be either fed directly into the pipeline orliquefied and conveyed from a first CO₂ pump 27 e.g. into a pipeline(not shown). The first CO₂ pump 27 raises the pressure of the liquid CO₂to the pressure given in the pipeline.

Going back to the offgas 23 it can be seen that the offgas streamsthrough the second heat exchanger 17 and the first heat exchanger 11,thereby taking up heat from the flue gas stream 3. At the exit of thefirst heat exchanger 11 the offgas 23 has a temperature of approximately26° C. to 30° C. and a pressure of approximately 26 bars (cf. flag no.16).

For maximising the energy recovery it is known to overheat the offgas 23with an offgas superheater 29 and then convey it to a expansion turbine31 or any other expansion machine. Wherein mechanical energy is recycledand afterwards the offgas is emitted into the surroundings with a lowpressure approximately corresponding to the surrounding pressure.

This installation described by means of FIG. 1 for liquefying CO2 isrelatively simple and works without problems. The disadvantage of thisprior art production of liquid CO₂ out of flue gas of power plants e.g.fuelled with fossils is its high energy demand having negative effectson the net efficiency degree of the power plant.

Thus the invention has the object to provide a method and aninstallation for liquefying the CO₂ contained in the flue gas operatingwith a reduced energy demand and thus increasing the net efficiencydegree of the power plant.

At the same time the method should be as simple as possible and theoperation technique favourably controllable in order to guarantee arobust and trouble-free operation.

According to the invention this object is solved with a method forproducing liquid CO2 out of combustion flue gases wherein the flue gasis partially condensed in a single stage phase separation, the singlestage phase separation comprising at least one heat exchanger and aseparation drum, wherein the at least one heat exchanger is cooled byexpanded offgas and expanded liquid CO2 and wherein a part of the CO2 isexpanded to a first pressure level and is separated after having passedthe at least one heat exchanger into liquid CO2 and gaseous CO2 in anadditional separation drum, wherein the gaseous CO2 and the liquid CO2of the additional separation drum are expanded to a second pressurelevel. A second part of the liquid CO2 of the separation drum isexpanded to a third pressure level for cooling the CO2 in the at leastone heat exchanger.

Due to the reduced volume flow resulting from evaporation of the CO₂ ata higher pressure level the result is a considerable reduction of therequired power for the second compressor 25 having the direct effect ofan improved net efficiency degree of the upstream power plant.

A further advantageous embodiment of the claimed invention comprises thestep that the pressure of a third part of the liquid CO2 of the firstseparation drum is raised to a fourth pressure level for cooling the CO2in the at least one heat exchanger.

This CO₂ stream then can be fed to the compressor 25 at an even highercompression stage resulting in a further reduced power consumption.

It is preferred that the second part of the liquid CO2 of thisseparation drum is expanded to a pressure of approximately 15 bar to 25bar, preferably to 20 bar. This pressure range matches with the commoncompression ratios usually applied for centrifugal compressors.

A further advantageous embodiment of the claimed method comprises thatthe third part of the liquid CO2 of the first separation drum is raisedto a pressure of approximately 40 bar to 50 bar, preferably to 45 bar.

These pressure levels allow an energy efficient operation of the planton the one hand while keeping commercially available compression ratiosand allow to run the plant at different operating points depending forexample on the required quality of CO2 and/or ambient temperature.

It is also advantageous to use the partial streams of CO2 from theseparation drums for cooling purposes in the at least one heatexchanger.

By using these CO2 streams for cooling purposes it can be avoided to useflammable cooling media, which results in a reduced danger of fire andminimizes the costs for security systems.

By feeding the CO2 streams to different stages of a second compressordepending on their pressure level a reduction of the energy consumptionis achieved.

Compressing the flue gas (3) in a first compressor and then cooling itin a first cooler and/or drying it in a drier before entering the atleast one heat exchanger reduces the volume of the flue gas, since mostof the water vapour has been separated. This means that the size of thedrier and the plant for producing liquid CO₂ can be smaller resulting inreduced energy losses and reduced costs.

By expanding the offgas from the last separation stage to approximately27 bar and resulting in a temperature of approximately −54° C. beforeentering the at least one heat exchanger the pressure level afterexpansion is as high as possible thus maximizing the energy recovery inthe expander.

A further reduction of the energy consumption can be achieved byexpanding the offgas after having passed the at least heat exchanger inat least one expansion machine and subsequently feeding it again to theat least one heat exchanger.

Optionally the offgas 23 can be superheated after having passed the atleast heat exchanger and before entering the at least one expansionmachine. If waste heat can be used for superheating, the output of theexpansion machine can be increased resulting in an better overallefficiency of the plant.

Preferably two expansion stages are to be used (c. f. FIG. 3) thusmaximizing the amount of CO₂ that can be directed to the third andfourth pressure level.

Further advantages of the claimed invention are explained in connectionwith FIGS. 2 and 3 in the following.

DRAWINGS

Shown are in:

FIG. 1 an installation for CO₂ liquefaction out of flue gases accordingto the prior art and

FIGS. 2 and 3 embodiments of installations for CO₂ liquefactionaccording to the invention.

DESCRIPTION OF THE DRAWINGS

In FIG. 2 identical components are designated with identical referencenumerals. The statements concerning FIG. 1 correspondingly apply.

The treatment of the flue gas stream 3 in the first compressor 1, thefirst cooler 5, the drier 7, the first heat exchanger 11 takes place asdescribed by means of FIG. 1. The flue gas stream 3 flows from the firstheat exchanger 11 directly to the second heat exchanger 17 and is thenconveyed to the now first separation drum 19. The two phases (liquid andgaseous) of the flue gas stream 3 are divided in the first separationdrum 19 into the offgas stream 23 and a partial stream of liquid CO₂. Atthe bottom of the first separation drum 19 this partial stream isextracted and has the reference numeral 3.3 such as in FIG. 1.

As already explained in the description of FIG. 1, the partial stream3.3 is expanded to a pressure of 17.5 bar in a third pressure reducingvalve 15.3, thereby cooling down to −54° C. The partial stream 3.3streams through the second heat exchanger 17, thereby taking up heatfrom the flue gas stream 3 and enters with a temperature ofapproximately −47° C. (cf. flag no. 8′) into a second separation drum33.

There the partially liquid and partially gaseous CO₂ has a pressure ofapproximately 16.5 bar and a temperature of −47° C. (cf. flag no. 9′).

In the head of the second separation drum 33 the gaseous phase isextracted and expanded in a fourth pressure reducing valve 15.4. Thegaseous partial stream being extracted at the head of the secondseparation drum 33 is designated with reference numeral 3.4 in FIG. 2.

At the bottom of the second separation drum 33 a liquid stream 3.5 isextracted and expanded in a fifth pressure reducing valve 15.5.Subsequently the partial streams 3.4 and 3.5 are brought together again.Then they have a pressure of approximately 5 to 10 bar and a temperatureof −54° C. (cf. flag no. 7 d′).

A second portion 3.6 of the CO₂ from the first separation drum 19 isexpanded via a sixth pressure reducing valve 15.6 to a pressure of ≈23bar (c. f. flag 7 e′) and returned to the exchanger 17 at anintermediate entry point.

With this partially liquid, partially gaseous CO2 the flue gas stream 3in the second heat exchanger 17 is cooled.

As the entrance temperature of the partial stream 3.6 is higher than theentrance temperatures of the offgas 23 as well as the partial stream3.3, the flue das stream 3 first is cooled with the partial stream 3.6.Thus it is possible to take up heat from the flue gas stream 3 even withthis higher temperature of −45° C. In FIG. 2 this fact is illustrated bythe position of the heat exchanging area of the partial stream 3.6.

The partial stream 3.6 leaves the second heat exchanger 17 with atemperature of approximately −22° C. to −29° C. (cf. flag no. 13′) andis then conveyed directly the first heat exchanger 11. In the first heatexchanger 11 the partial stream 3.6 takes up heat from the flue gasstream 3. The partial stream 3.6 leaves the first heat exchanger (cf.flag no. 11′) with a temperature of approximately 25° C. and a pressureof approximately 18 bar and can thus be conveyed to the secondcompression stage of the second compressor 25.

As the partial stream 3.6 can be conveyed to the second compressionstage of the second compressor 25, the partial stream 3.3, which has tobe conveyed to the first compression stage of the second compressor 25,is correspondingly reduced. Consequently the power required by thesecond compressor 25 is smaller. This has positive effects on the energydemand of the installation according to the invention.

Occasionally the remainder 3.7 of the liquid CO₂ from the firstseparation drum 19 is pumped (c. f. ref. number 37) to a pressure of 45bar (c. f. flag 7 g) with the CO₂ pump 37 and returned to exchanger 17also at an intermediate entry point.

Parallel to the partial stream 3.6 a further partial stream 3.7 flowsthrough the second heat exchanger 17 and the first heat exchanger 11.The partial stream 3.7 is driven by a CO2 pump 37 and brought to anincreased pressure level of approx. 45 bar (cf. flag no. 7 g). An eighthvalve 15.8 serves to control the amount of CO2 that is pumped by the Co2pump 37.

As the entrance temperatures of the partial streams 3.6 and 3.7 arehigher than the entrance temperatures of the offgas 23 as well as thepartial stream 3.3, the flue gas stream 3 first is cooled with thepartial streams 3.6 and 3.7. Thus it is possible to take up heat fromthe flue gas stream 3 even with the a. m. higher temperature. In FIG. 2this fact is illustrated by the position of the heat exchanging area ofthe partial stream 3.7.

The partial stream 3.7 leaves the second heat exchanger 17 with atemperature of approximately −22° C. to −29° C. (cf. flag no. 20) and isthen conveyed directly the first heat exchanger 11. In the first heatexchanger 11 the partial stream 3.7 takes up heat from the flue gasstream 3. The partial stream 3.7 leaves the first heat exchanger (cf.flag no. 21) with a temperature of approximately 25° C. and a pressureof approximately 44 bar and can thus be conveyed after the second andbefore the third compression stage of the second compressor 25.

As the partial stream 3.7 can be conveyed to the third compression stageof the second compressor 25, the partial stream 3.3, which has to beconveyed to the first compression stage of the second compressor 25, iscorrespondingly reduced. Consequently the power required by the secondcompressor 25 is smaller. This has positive effects on the energy demandof the installation according to the invention.

Extraction of partial stream 3.7 is possible when the off-gas energy isused by at least double expansion via expanders 31 and 39 as shown inFIG. 3. This maximizes the cold recovery from the off-gas as describedlater.

All liquid or two phase CO₂ streams (3.3, 3.6., 3.7) are evaporated inexchanger 17 and 11 before being sent to CO₂ recompressor or secondcompressor 25. Depending on the pressure level the CO₂ streams are fedat different compression stages of the second compressor 25.

Using different pressure levels for the evaporation of the CO₂ hasseveral advantages: It gives better control over the flue gascondensation. Furthermore the overall compression requirements can beminimized having CO₂ at elevated pressures readily available.

A further possibility of reducing the energy demand of the CO₂liquefaction plant can be seen in not only overheating the offgas 23 inthe offgas superheater 19 after the exit from the first heat exchanger11, but also reconvey it to the second heat exchanger 17 after theexpansion in the expansion turbine 31. After the overheating the offgashas a temperature of approximately 80° C. to approximately 100° C. witha pressure of approximately 26 bar (cf. flag no. 17). By the expansionin the first expansion machine 31 the pressure drops to 2.3 bar and theoffgas 23 reaches a temperature of −54° C. Thus the offgas 23 can oncemore contribute to the cooling of the flue gas stream 3 resp. thepartial stream 3.2. Afterwards the offgas 23 can be emitted to thesurroundings with a low pressure and approximately surroundingtemperature.

It is also possible to carry out a multi-stage expansion and overheatingof the offgas 23 as is shown in FIG. 3.

In the embodiment shown in FIG. 3 the offgas 23 is but sent directlyafter the exit from the first heat exchanger 11 to the first expansionturbine 31 and further to the second heat exchanger 17. From the secondheat exchanger 17 the offgas flows through the first heat exchanger 11.Before entering the first expansion turbine 31 the offgas has atemperature of approximately 30° C. with a pressure of approximately 26bar (cf. flag no. 16). Due to the expansion in the first expansionmachine 31 the pressure drops to 8 bar and the offgas reaches atemperature of −54° C.

The second stage of expansion comprises a second expansion turbine 39.Before entering the second expansion machine 39 the offgas 23 hastemperature of approximately 30° C. (cf. flag 22). Due to the expansionin the second expansion machine 39 the pressure drops to 2 bar and theoffgas reaches a temperature of −47° C. (cf. flag 23).

Thus the offgas 23 can once more contribute to the cooling of the fluegas stream 3 resp. the partial stream 3.2. Afterwards the offgas 23 canbe emitted to the surroundings with a low pressure and approximatelysurrounding temperature.

The single or multi-stage expansion as well results in a considerablereduction of the energy demand of the installation according to theinvention, as on the one hand the offgas 23 contributes to a greateramount to the cooling of the flue gas stream 3 resp. the partial stream3.2 and the expansion machine 31 and/or 39 generate mechanical work,which e. g. can be used for driving the first compressor 1 or the secondcompressor 25. All in all it can be stated that the method according tothe invention and the installation for CO₂ liquefaction required forcarrying out the method according to the invention are still relativelysimple in their design in spite of the considerable advantages.

Furthermore, this setup clearly improves the control over the flue gascondensation. With adjustment of the flow rate over the CO₂ pump 37 andthe valves 15. 6 and 15.3 the driving force for heat transfer, theLogarithmic Mean Temperature Difference (LMTD), is varied. In this waythe performance of the separation stage can be adjusted. This isespecially important, when operating at condensation temperatures nearthe sublimation and freezing point of CO₂.

In order to maximize the described effect, the heat recovery out of theoffgas from separation can be increased by having the vent gas/offgas 23recirculated to the cold box after expansion, at least once beforereleasing it to the atmosphere.

Table of flags, pressures and temperatures. Temperature, approx.Pressure, approx. Flag no. [° C.] [bar]  1 13 35.7  2 13 35  5 −19 34.7 5′ −19 34.7  6 −51 34.3  6′ −51 34.3  7¹ −54° C. 27  7a −54 17  7a′ −5427  7b −54 5 to 10  7b′ −48 44  7c −54 17.5  7c′ −54 17.5  7d −54 5 to10  7d′ −54 5 to 10  7e −45 ≈20 to 23  7g −47 45  7h −47 44  8 −47 16.5 8′ −47 16.5  9 −47 16.5  9′ −47 16.5 10 −22 to −29 20.5 11 25 20 11′ 26to 30 19 12 −7 5-10 12′ −7 5 to 10 13 −22 to −29 20 14 −22 to −29 5-1016 26 to 30 26 17 80 to 100 25.8 18 −54 2.3 19  80 to 130 60 to 110 20−22 to −29 43.5 21 26 to 30 43 22 26 to 30 7 The tolerances for Thetolerances for the temperatures the pressures are ±5° C. are ±5 bar ¹Istdas Offgas 23 in FIG. 1

1. A method for producing liquid CO₂ combustion flue gases partiallycondensed in a single stage phase separation, comprising: cooling byexpanded offgas and expanded liquid CO₂ at least one heat exchanger anda separation drum; separating the expanded CO₂ as liquid CO₂ and gaseousCO₂ after having passed the at least one heat exchanger and into anadditional separation drum; expanding the gaseous CO₂ and a first partof the liquid CO₂ of the additional separation drum to a first pressurelevel; and expanding the pressure of a second part of the liquid CO₂ ofthe additional separation drum to a second pressure level for coolingthe CO₂ in the at least one heat exchanger.
 2. The method according toclaim 1, further comprising raising a pressure of a fourth part of theliquid CO₂ of the first separation drum to a fourth pressure level forexpanding and cooling the CO₂ in the at least one heat exchanger.
 3. Themethod according to claim 2, wherein the fourth part of the liquid CO₂of the additional separation drum is raised to a pressure ofapproximately 40 bar to 50 bar, or to 47 bar.
 4. The method according toclaim
 1. wherein partial streams of CO₂ from the separation drums areused for cooling the at least one heat exchanger.
 5. A method accordingto claim 1, wherein the CO₂ streams are fed to different stages of asecond compressor depending on pressures of the CO₂.
 6. A methodaccording to claim 1, wherein the flue gas is compressed in a firstcompressor, cooled in a first cooler and/or dried in a drier beforeentering the at least one heat exchanger.
 7. A method according to claim1, further comprising expanding offgas from a last separation stage toapproximately 27 bar and a temperature of approximately −54° C. beforeentering the at least one heat exchanger.
 8. A method according to claim1, further comprising expanding the offgas after passing the at leastone exchanger in at least one expansion machine; and subsequentlyfeeding the offgas again to the at least one heat exchanger
 17. 9. Amethod according to claim 1, further comprising superheating offgas in asuperheater after passing the at least one heat exchanger and beforeentering the at least one expansion machine.
 10. A plant for producingliquid CO₂ from partially condensed combustion flue gases comprising: atleast one heat exchanger, separation drums, several pressure reducingvalves and a second multi-stage compressor.
 11. A plant according toclaim 10, further comprising at least one CO₂ pump.
 12. A plantaccording to claim 10, further comprising at least one expansionmachine.
 13. A plant according to claim 10, further comprising at leastone offgas superheater.
 14. A plant according to claims 10, furthercomprising a first compressor, a first cooler and a drier.